A branched optical communication system may include a relatively long trunk path that may be terminated at a transmitting and/or receiving trunk terminal, and at least one branch terminal coupled to the trunk path through a branch path and a branching unit (BU). Each BU may include one or more optical add/drop multiplexers (OADM). Channels or wavelengths of a wavelength division multiplexed (WDM) signal may be added to and/or dropped from the trunk path via the OADMs to direct optical signals on selected channels to and from the branch terminals.
In long distance optical communication systems, such as an undersea optical cable system, it may be advantageous to monitor the health of the system and particularly the optical paths in the system. For example, line monitoring systems (LMS) can be used to detect faults or breaks in the optical transmission cable, faulty repeaters or amplifiers or other problems with the system. LMS include use of optical time domain reflectometry (OTDR) and high-loss loop-back (HLLB) equipment and techniques.
In general, these LMS systems include line monitoring equipment (LME) that generates a monitoring, or test, signal, e.g. noise or tones, representing a pseudo random bit sequence. The line monitoring equipment may launch the monitoring signal into the outbound path with the information signals. Some portion of the monitoring signal may be returned to the LME and used as a metric to determine the health of the system.
According to an OTDR LMS technique, for example, elements in the outbound path may reflect (e.g., backscatter) portions of the monitoring signal. Backscattered monitoring signal portions may be returned (e.g., on the same outbound path or a different path such as the inbound path) and detected in an OTDR receiver of the LME. The transmission characteristics of each element in the path may also affect the amount of signal reflected at points after that element, for example, by attenuating the monitoring signal or the reflected signal. The magnitude of the backscattered or reflected signal from each element or point along the optical path may be representative of the health of the system.
According to a high-loss loop-back (HLLB) LMS technique, the monitoring signal may be returned to the LME from each coupling point in the path pair through a HLLB path within an amplifier or repeater. The LME may then separate the returned monitoring signals from the data signals, and process the returned monitoring signals to obtain data representing the HLLB loop gain imparted to the monitoring signal in its propagation from the line monitoring equipment, through the HLLB and any intervening optical paths and amplifiers, and back to the line monitoring equipment. Loop gain is the ratio of the magnitude of the detected sample from a given coupling point to the magnitude of the monitoring signal launched into the outbound path. Deviations in HLLB loop gain may indicate a fault in the system.
A challenge associated with LMS operation in branched optical networks is that the monitoring signals can simultaneously return from different network paths, e.g., trunk and branch, making it difficult to assign location to the returning signals. In addition, it may be difficult to interpret loop-gain changes in branched networks because a branch fault can affect trunk loop-gains and a trunk fault can affect branch loop-gains. Thus, within a branched optical network, it may be difficult with present LMS methods to localize and classify a system fault from a single isolated LMS measurement.